[LICM] Ensure LICM can hoist invariant.group
Invariant.group's are not sufficiently handled by LICM. Specifically,
if a given invariant.group loaded pointer is not overwritten between
the start of a loop, and its use in the load, it can be hoisted.
The invariant.group (on an already invariant pointer operand) ensures
the result is the same. If it is not overwritten between the start
of the loop and the load, it is therefore legal to hoist.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D144053